This invention relates to a particle collection apparatus and method for sampling sub-micron diameter particles which are airborne.
This invention may be adapted for use as a pollution detection device, as an experimental tool for analyzing particles which have hitherto not been measured accurately, and for scientific measurement of the atmosphere for determining types and sizes of particles contained in jet and rocket exhaust.
It is well-known to use filters to extract particles of greater than one micron diameter from gas samples to be tested. The commonly used glass fiber filters and HEPA filters effectively trap airborne particles having diameters greater than or equal to 0.3 micron. However, these filters are unsuitable for collecting airborne particles of less than 0.3 micron in diameter because such particles exhibit Brownian movement and behave more like gases than particles.
Many attempts have been made to sample particulate matter contained as airborne particles in air. This is especially true where air pollution measurements are desired of smokestack emissions, automobile emissions, jet engine exhaust, dispersion of contaminants from a source of contamination through the atmosphere, and any other gases which are of interest to the researcher. The article, Air Sampling Instruments For Evaluation Of Atmospheric Contaminants, Fifth Edition, 1978, American Conference of Governmental Industrial Hygienists, pp. A-5 to A-9, sets forth a variety of methods of sampling for gases and vapors. One typical method which is commonly used is absorption. In this process, a gas sample is bubbled through an absorbing liquid. This is sometimes called a gas washing. Another type mentioned in the article is condensation or freeze-out. This involves a trap immersed in a low temperature bath such as dry ice and acetone or liquid nitrogen. The gas sample is caused to flow through the trap and moisture and other types of contaminants are collected by freezing out or liquefying. It is possible to use a combination of methods with this process for collecting a specific material. The materials thus collected may be analyzed by means of a gas chromatograph or a mass spectrometer. This article, however, does not teach or suggest a specific procedure for obtaining specific sizes of particles, in particular sub-micron diameter particles having a diameter of less than or equal to 0.3 micron. Also, although the use of a trap is suggested in the above article, no specific apparatus is suggested and no particular procedure of obtaining sub-micron sized particles is taught.
Use of metal-coated Nuclepore filters is discussed at some length by the article by W. Hollander, J. Schormann, W. Stober, F. J. Monig, and N. Schwarzer, entitled "Sensitive Integrated and Time-Resolved Aerosol Measurement by Means of Light-Transmission Changes of Metal-Coated Nuclepore Filters (Filter-Fotometer)", Environ. Sci. & Technol., 1981, 15(4), pp. 471-473. This article is incorporated herein by reference. Various types of Nuclepore filters are discussed, and the theory of operation is set forth. Such filters are useful for measuring particulate air pollutants. One type of measurement involves the change in the light transmissivity through the filters caused by aerosol particles deposited therein. There does not appear to be any suggestion of crushing the Nuclepore filter and subjecting it to further processing so as to be able to analyze the sub-micron diameter particles trapped therein, nor is there suggestion of passing particle-laden water therethrough to extract sub-micron diameter particles therefrom.
A variety of types of particle sampling and measurement devices are discussed in W. E. Farthing, "Particle Sampling and Measurement", Environ. Sci. & Technol., 1982, 16(4), pp 237A-244A. Here, a particular apparatus for obtaining a gas sample and analyzing the same is shown. A variety of types of applications for particle sampling devices is discussed in the above article. A variety of sampling methods are discussed, including the hitherto undiscussed inertial sampling methods. Optical methods are also discussed, and other types of particle detection devices are pointed out without elaboration. Nonetheless, the above-identified article does not show a specific type of apparatus used to obtain sub-micron diameter particles. Furthermore, the use of Nuclepore membranes or filters is only discussed broadly and without reference to any specific method or apparatus of obtaining sub-micron diameter particles.
A specific type of experimental procedure employing quartz filters is discussed in the article by P. Solomon, M. Derrick, J. Moyers, and P. Hyde, entitled "Performance Comparison of Three Samplers of Suspended Airborne Particulate Matter", Journal of the Air Pollution Control Assoc., 1982, 32(4), pp. 373-375. In this article, quartz filters are discussed as being used in place of glass fiber filters. A constant air flow or gas flow is maintained through the quartz filters. The filters used then are tested to determine the amount of particulates collected and to determine concentrations of various elements or chemicals which make up the particulate mass. An ethanol solution is used to wet the filters so as to extract particulate matter collected therein. Ion chromatography can then be used to determine the exact composition of the particulate matter in the extraction solution. The extracted solutions can also be analyzed by flame atomic adsorption spectroscopy. However, there is no teaching or suggestion of using another type of apparatus to obtain those particles of sub-micron diameter which ordinarily escape collection by quartz or fiber filters. Also, there is no teaching or suggestion of using a Nuclepore filter to extract particulate matter from gas which has already passed through a quartz or fiber filter.